1. Field of the Invention
The present invention relates generally to a photonic crystal optical waveguide structure for an optical communication system. More particularly, the present invention is directed to a ring photonic crystal optical waveguide fiber structure for creating an endlessly single mode optical waveguide.
2. Technical Background
The investigation of properties of specific optical fiber designs has continued to keep pace with an ever increasing demand for high capacity, long--haul waveguide fiber. Data transmission rates in the terabit range are being studied and communication systems having regenerator spacings greater than 100 km are under consideration. The requirement in the telecommunication industry for greater information capacity over long distances, without regenerators, has led to a reevaluation of single mode fiber index design.
Optical waveguide fibers can be generally classified into single-mode fiber and multimode fiber. Both types of optical fiber rely on total internal reflection (TIR) for guiding the photons along the fiber core. Typically, the core diameter of single-mode fiber is relatively small, thus allowing only a single mode of light wavelengths to propagate along the waveguide. Single-mode fiber can generally provide higher bandwidth because the light pulses can be spaced closer together and are less affected by dispersion along the fiber. Additionally, the rate of power attenuation for the propagating light is lower in a single-mode fiber.
Optical fibers having a larger core diameter are generally classified as multimode fibers, and allow multiple modes of light wavelengths to propagate along the waveguide. The multiple modes travel at different velocities. This difference in group velocities of the modes results in different travel times, causing a broadening of the light pulses propagating along the waveguide. This effect is referred to as modal dispersion, and limits the speed at which the pulses can be transmitted; in turn limiting the bandwidth of multimode fiber. Graded-index multimode fiber (as opposed to step-index multimode fiber) has been developed to limit the effects of modal dispersion. However, current multimode and graded-index multimode fiber designs still do not have the bandwidth capabilities of single-mode fiber.
Photonic crystals are another means by which photons (light modes) can be guided through an optical waveguide structure. Rather than guiding photons using TIR, photonic crystals rely on Bragg scattering for guiding the light. The characteristic defining a photonic crystal structure is the periodicity of dielectric material along one or more axes. Thus, photonic crystals can be one-dimensional, two-dimensional and three-dimensional. These crystals are designed to have photonic band gaps which prevent light from propagating in certain directions within the crystal structure. Generally, photonic crystals are formed from a periodic lattice of dielectric material. When the dielectric constants of the materials forming the lattice are different, and the material absorbs minimal light, the effects of scattering and Bragg diffraction at the lattice interfaces allow the photons to be guided along or through the photonic crystal structure.
An exemplary photonic crystal 10 which is periodic in two directions and homogeneous in a third is shown in FIG. 1. More specifically, photonic crystal 10 comprises a triangular lattice of dielectric columns 12, extending in the Z-axis direction, which are periodic in the X-axis and Y-axis directions (measured center to center). The photonic crystal 10 is assumed to be homogeneous in the Z-axis direction. As a result of this structure, photonic band gaps are created in the plane of periodicity (X and Y planes).
In the exemplary photonic crystal 10, the light modes will usually be oscillatory in the Z-axis direction, because the crystalline structure is homogeneous along the Z-axis.
It is also generally assumed that the light modes will only propagate through the photonic crystal in the X-Y plane. The photonic crystal 10 is also invariant under reflections through the X-Y plane. This mirror symmetry allows the modes to be classified into two distinct polarizations, namely transverse electric (TE) and transverse magnetic (TM) modes.
It is also known that a defect can be introduced into the crystalline structure for altering the planar propagation characteristics and localizing the light modes. For example, photonic crystal 10 includes a central column 14 (shown as a solid black column) comprising a dielectric material that is different from the other periodic columns 12. Additionally, the size and shape of central column 14 can be modified for perturbing the single lattice site. As a result, a single localized mode or set of closely spaced modes is permitted having frequencies within the band gap.
The characteristics of the crystalline structure produce a photonic band gap (PBG). The defect in the crystalline structure allows a path for light to travel through the crystal. In effect the central column 14 creates a central cavity which is surrounded by reflecting walls. Light propagating through the central column 14 (along the Z-axis direction) becomes trapped within the resulting photonic band gap and cannot escape into the surrounding periodic columns 12. Thus it has been demonstrated that light, whether a pulse or continuous light, can also be guided through this type of photonic band gap crystal.
An optical waveguide fiber having a photonic band gap crystal cladding region known within the prior art is shown in FIG. 2. The photonic crystal fiber (PCF) 16 includes a porous clad layer 18, containing an array of air voids 20 that serve to change the effective refractive index of the clad layer 18. This in turn serves to change the properties of the fiber 16 such as the mode field diameter or total dispersion. The air voids 20 defining the clad layer 18 create a periodic matrix around the central fiber core 22, usually formed from solid silica. The distribution of light power across the waveguide (mode power distribution) effectively determines the properties of the optical waveguide. Changing the effective index of the clad layer 18 changes the mode power distribution and thus the properties of the PCF optical waveguide 16.
The manufacture of a porous clad PCF 16 having an array of voids or pores 20 as shown is difficult because the porosity volume and distribution must be controlled in the preform. Further, the control of the PCF clad layer porosity must be maintained during drawing of the preform down to the dimensions of a waveguide fiber, such as PCF 16. The drawing step occurs at very high temperatures and the final fiber diameter is small, about 125 .mu.m. The drawing step must therefore include maintaining a precise balance of pressure within the pore 20 against viscous forces of the materials surrounding the pore 20 under relatively extreme conditions.
Another problem associated with the porous clad PCF, such as PCF 16 is illustrated in FIG. 3. More specifically, FIG. 3 shows the resulting structure of the air voids 20 forming the clad layer 18 after either fusion splicing or tapering. As shown, the size of the pore forming each air void 20 becomes constricted or closed which in turn adversely affects the properties of the fiber 16 at the splice or tapered region. Accordingly, PCF 16 is not well suited for applications in which the fiber must be spliced or tapered.
However, an advantage realized through PCF structures is that the large contrast between core and clad effective index afforded by these structures can be used to provide large effective area, thereby mitigating non-linear effects on transmitted signal integrity. In view of the advantages associated with photonic crystal fiber structures, it is desirable to provide an optical waveguide PCF which reduces the problems of "closed" air gaps within the fiber splicing region, as well as overcoming the additional problems described above and known with prior photonic crystal fiber designs.